Crosslinking and direct binding studies have confirmed our previous suggestion that chromatin core particles can bind excess histones as octamers in 0.6 M NaCl. Acidic proteins, such as polyglutamic acid, also bind histones as octamers, even at physiologic ionic strength. This interaction allows the complete reassembly of SV-40 minichromosomes in a cooperative fashion in less than 60 min, a marked contrast to slower and incomplete assembly in the absence of the polyanion. We have constructed chromatin and core particles from the four smaller histones and synthetic polydeoxyribonucleotides. The complementary homopolymers do not form nucleosomes. The alternating copolymers form chromatin very similar to native material. Due to lack of sequence heterogeneity in the DNA, core particles derived from these chromatins are ideally suited for high resolution studies of histone-DNA interactions. A number of features of core particle structure, conformational transitions, and DNA geometry have been revealed or defined with much higher precision using these semi-synthetic core particles.